The stock of genuine excellence is thin in this year's O. Henry Award collection. Editor Abrahams has rightly chosen Cynthia Ozick's stark, chillingly deliberate story of mother-child annihilation in a Nazi death camp, ""The Shawl,"" as the first prize winner. There are no runners-up awards, although Tobias Wolff's ""In the Garden of the North American Martyrs,"" W. D. Weatherell's ""The Man Who Loved Levittown,"" and Lee Smith's ""Between the Lines"" would probably be the most deserving. Well-known fictionists like Joyce Carol Oates, Paul Theroux, Kay Boyle, Jack Matthews, Alice Walker, and John Irving are represented by solid but unexciting stories. (The Irving in particular seems as if just recently tapped from a mold: another cozy yet angst-y comedy about educated young marrieds in--where else?--a college town.) And newcomers Barbara Reid (""The Waltz Dream"") and Steve Stern (""Isaac and the Undertaker's Daughter"") may be sprawling or excessively knotty with their talents, but they do make one eager for more from them. As for the rest, it mostly runs the gamut from A to B, with much that is trite and feather-weight. On balance, then, a particularly tame and unprovocative collection--the weakest O. Henry showing in recent, years.